At one time an Onan generator built for RVs just didn't fit under a Class B though I knew of one DIY N1T Sprinter that accomplished it. I have no idea whether that is true today. Until recently diesel Class Bs seem to have propane anyway.
2015 Advanced RV Ocean One Mercedes Benz Sprinter
Previous Class Bs:
2011 Great West Van Legend Sprinter
2005 Pleasure-way Plateau TS Sprinter